1. Field of the Invention
The present invention relates to an apparatus for forming fine patterns in semiconductor devices, display devices and micro-electro-mechanical systems and more particularly to an image projecting system using an optical component, which is made of birefringent material, in the lithography techniques.
2. Description of the Related Art
Generally, a photoresist film has been used as a photosensitive material in lithography processes. A conventional photosensitive material for a camera is shown in FIG. 1A, wherein the remaining photosensitive material after the development has a characteristic to increase in density as the intensity of the illumination increases. Accordingly, when an image is formed on a film by a lens system such as a camera, the contrast of the image depreciates. In photosensitive material used in lithography processes, the density of the material remains substantially unchanged changed within a restricted region even if light intensity increases. However, its density may dramatically increase at the light intensity increases over a critical value, as shown in FIG. 1B. Accordingly, when an image is formed on this photosensitive material, the boundary between a photosensitive material region and a non-photosensitive material region is remarkable because of the high contrast on the image. These characteristics enhance the resolution of the patterns to be used in semiconductor devices and display devices.
In the conventional lithography apparatus, after an image is converged into a point through an optical lens system, the image is spread out again. In this case, the light intensity is weak with the spread of image so that the photosensitive material can not be sensitized below a predetermined light intensity. Although the illumination has a light intensity capable of sensitizing the photosensitive material, the desired resolution can not be obtained on the photosensitive material when the image is spread out in the direction of the axis of the optical lens system according to the intensity of the spread light. In view of these two features, the range of image projection which satisfies a desired resolution is called the "depth of focus." Since the depth of focus is in proportion to the wavelength of light and in inverse proportion to the square of the numeral aperture (NA) which indicates the relative size of the optical lens system, the margin of the depth of focus becomes narrower with the short wavelength of light, thereby reducing the yield.
FIGS. 2A and 2B are schematic diagrams showing the characteristics of isotropic optical components in a conventional optical lens system, wherein the isotropic optical components have the same refractive index. The isotropic optical component 110 (FIG. 2A) made of an isotropic material or an isotropic optical unit 100 (FIG. 2B) consisting of the isotropic optical component 110 form an image point 115 along the optical axis 1 of the optical lens system, irrespective of an one-directional linear polarization 112 or a vertical linear polarization 113 vertical to the one-directional linear polarization 112. The light converged into the image point 115 is spread out after passing through the image point 115. Accordingly, the image can be obtained within a limited region taken along the axis, which has the intensity required to sensitize the photosensitive material, and this limited region is also called the depth of focus 117.
The isotropic optical component 110 or the isotropic optical unit 100 has a low resolution because the depth of focus 117 in order to have a needed resolution is narrow. To achieve the margin of the desired depth of focus, the conventional lithography process has used a photosensitive material of which only the surface is sensitized or used a mechanical method which moves a wafer along the optical axis using an upward or downward moving stage in order that several exposure processes are carried out for extending the depth of focus. Also, the depth of focus has been extended by attenuating the cone caused by the light flux converged into the image point and by fabricating a photomask with a phase-shifting material which provides phase difference between the patterns.
The first method of the conventional techniques needs the additional process for changing the characteristics of the photosensitive film and a special material is needed to form such a photoresist film. Therefore, it has the problem in that the process becomes more complicated and the manufacturing cost is increased.
The second method of the conventional techniques has merit in that the conventional process and photosensitive material are used. However, it also has the problem in that the exposure must be performed several times with the movement of the wafer in the direction of the axis of the optical lens system, therefore, the optical lens system is not stable during the wafer movement. As a result, it needs a long process time because the expose must be performed several times. Moreover, as the area to be exposed is extended, the exposing method is changed to one in which the stage moves horizontal direction with exposure so that the second method can not be used any more. Also, the third method of the conventional technique has a lot of problems in that it is very difficult to fabricate the needed photomask and the image produced by such a photomask is unsymmetrical in the cone type light flux of light converged to an focusing point so that a lot of aberration may occur.